Protected light source for multiple wavelength division multiplexed passive optical networks (WDM-PONs)

ABSTRACT

A protected light source for generating seed light for at least two wavelength division multiplexed passive optical networks (WDM-PONs). The protected light source includes an optical coupler having N≧2 input ports and M≧2 output ports, each output port being optically connected to supply seed light to a respective set of one or more WDM-PONs. A respective multi-wavelength light source (MWLS) is optically coupled to supply seed light to each input port of the optical coupler. A controller unit controls operation of each multi-wavelength light source (MWLS).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 12/412,743 filed Mar. 27, 2009 which claims benefitof U.S. Provisional Patent Application Ser. No. 61/040,224 filed Mar.28, 2008. The entire content of both applications is hereby incorporatedherein by reference.

MICROFICHE APPENDIX

Not Applicable.

TECHNICAL FIELD

The present invention relates to Passive Optical Networks, and moreparticularly, to a protected light source for multiple WavelengthDivision Multiplexed Passive Optical Networks (WDM-PONs).

BACKGROUND OF THE INVENTION

A passive optical network (PON) is a point-to-multipoint networkarchitecture in which unpowered optical splitters are used to enable asingle optical fibre to serve multiple premises. A PON typicallyincludes an Optical Line Terminal (OLT) at the service provider'scentral office connected to a number (typically 32-128) of OpticalNetwork Terminals (ONTs), each of which provides an interface tocustomer equipment.

In operation, downstream signals are broadcast from the OLT to the ONTson a shared fibre network. Various techniques, such as encryption, canbe used to ensure that each ONT can only receive signals that areaddressed to it. Upstream signals are transmitted from each ONT to theOLT, using a multiple access protocol, such as time division multipleaccess (TDMA), to prevent “collisions”.

A Wavelength Division Multiplexing PON, or WDM-PON, is a type of passiveoptical network in which multiple optical wavelengths are used toincrease the upstream and/or downstream bandwidth available to endusers. FIG. 1 is a block diagram illustrating a typical WDM-PON system.As may be seen in FIG. 1, the OLT 4 comprises a plurality oftransceivers 6, each of which includes a light source 8 and a detector10 for sending and receiving optical signals on respective wavelengthchannels, and an optical combiner/splitter 12 for combining lightfrom/to the light source 8 and detector 10 onto a single optical fibre14. The light source 8 may be a conventional laser diode such as, forexample, a distributed feed-back (DFB) laser, for transmitting data onthe desired wavelength using either direct laser modulation, or anexternal modulator (not shown) as desired. The detector 10 may, forexample, be a PIN diode for detecting optical signal received throughthe network. An optical mux/demux 16 (such as, for example, a Thin-FilmFilter—TFF) is used to couple light between each transceiver 6 and anoptical fibre trunk 18, which may include one or more passive opticalpower splitters (not shown).

A passive remote node 20 serving one or more customer sites includes anoptical mux/demux 22 for demultiplexing wavelength channels (λ1 . . .λn) from the optical trunk fibre 18. Each wavelength channel is thenrouted to an appropriate branch port 24 which supports a respectiveWDM-PON branch 26 comprising one or more Optical Network Terminals(ONTs) 28 at respective customer premises. In the WDM-PON of FIG. 1, oneof the branches 26 a includes a pair of ONTs 28, which share commonuplink and downlink wavelength channels using, for example, a timedivision multiple access scheme well known in the art. Typically, eachONT 28 includes a light source 30, detector 32 and combiner/splitter 34,all of which are typically configured and operate in a manner mirroringthat of the corresponding transceiver 6 in the OLT 4.

Typically, the wavelength channels (λ1 . . . λn) of the WDM-PON aredivided into respective channel groups, or bands, each of which isdesignated for signalling in a given direction. For example, C-band(e.g. 1530-1565 nm) channels may be allocated to uplink signalstransmitted from each ONT 28 to the OLT 4, while L-band (e.g. 1565-1625nm) channels may be allocated to downlink signals from the OLT 4 to theONT(s) 26 on each branch 26. In such cases, the respective opticalcombiner/splitters 12,34 in the OLT transceivers 6 and ONTs 286 arecommonly provided as passive optical filters well known in the art.

The WDM-PON illustrated in FIG. 1 is known, for example, from “Low CostWDM PON With Colorless Bidirectional Transceivers”, Shin, D J et al,Journal of Lightwave Technology, Vol. 24, No. 1, January 2006. With thisarrangement, each branch 26 is allocated a predetermined pair ofwavelength channels, comprising an L-band channel for downlink signalstransmitted from the OLT 4 to the branch 26, and a C-band channel foruplink signals transmitted from the ONT(s) 28 of the branch 26 to theOLT 4. The MUX/DEMUX 16 in the OLT 4 couples the selected channels ofeach branch 26 to a respective one of the transceivers 6. Consequently,each transceiver 6 of the ONT is associated with one of the branches 26,and controls uplink and downlink signalling between the ONT 4 and theONT(s) 28 of that branch 26. Each transceiver 6 and ONT 28 is rendered“colorless”, by using reflective light sources 8, 30, such as reflectivesemi-conductor optical amplifiers; injection-locked Fabry-Perot lasers;reflective electro-absorptive modulators; and reflective Mach-Zehndermodulators. With this arrangement, each light source 8, 30 requires a“seed” light which is used to produce the respective downlink/uplinkoptical signals. In the system of FIG. 1, the seed light for downlinksignals is provided by an L-band broadband light source (BLS) 34 via anL-band optical circulator 36. Similarly, the seed light for uplinksignals is provided by a C-band broadband light source (BLS) 38 via aC-band optical circulator 40. As is known in the art, the BLSs 34 and 38may be broadband light emitting sources that generate a continuousspectrum, such as a Light Emitting Diode (LED), or may be amulti-frequency laser source which generates a plurality of narrow-bandlights.

WDM-PONs suffer limitations in that the fibre trunk 18 and the BLSs34,38 constitute single points of failure of the entire network. Afailure of any one of these components effectively disconnects allsubscribers.

A typical method for implementing WDM-PON protection is to duplicate theWDM-PON system of FIG. 1, with both systems being connected to the sameONTs 26. A less expensive alternative, which provides light sourceprotection, is illustrated in FIG. 2. In the system of FIG. 2, theL-band and C-band BLSs 34 and 38 are protected by duplicating each BLS,and then routing light from a selected one of the two BLSs (for bothuplink and downlink seed light) using an optical switch 46, as shown inFIG. 2. While this arrangement does provide protection for the L-bandand C-band light sources, it also significantly increases the cost andcomplexity of the OLT 4.

SUMMARY OF THE INVENTION

An aspect of the present invention provides a protected light source forgenerating seed light for at least two wavelength division multiplexedpassive optical networks (WDM-PONs). The protected light source includesan optical coupler having N≧2 input ports and M≧2 output ports, eachoutput port being optically connected to supply seed light to arespective set of one or more WDM-PONs. A respective multi-wavelengthlight source (MWLS) is optically coupled to supply seed light to eachinput port of the optical coupler. A controller unit controls operationof each multi-wavelength light source (MWLS).

An advantage of the present invention is that distribution of light fromeach MWLS to all of the WDM-PONs is entirely passive, which reducescosts and improves reliability. In addition, a single protected lightsource can efficiently supply seed light to multiple WDM-PONs, whichallows higher quality (and thus higher cost) MWLSs to be used, withoutincreasing the cost of each WDM-PON, as compared to conventionalsolutions.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following detailed description, taken in combinationwith the appended drawings, in which:

FIG. 1 is a block diagram schematically illustrating a conventionalWDM-PON known in the prior art;

FIG. 2 is a block diagram schematically illustrating conventionaltechniques for protecting the fibre trunk and Broadband Light Sources ofthe WDM-PON of FIG. 1;

FIG. 3 is a block diagram schematically illustrating a protectedmulti-wavelength light source for multiple WDM-PONS in accordance with arepresentative embodiment of the present invention;

FIGS. 4 a-4 c schematically illustrate respective multi-wavelength lightsources usable in the embodiment of FIGS. 3; and

FIG. 5 is a block diagram schematically illustrating an Optical LineTerminal of a WDM-PON supplied using protected multi-wavelength lightsources of the type illustrated in FIG. 3.

It will be noted that throughout the appended drawings, like featuresare identified by like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a protected multi-wavelength light sourcefor multiple WDM-PONs. Representative embodiments are described belowwith reference to FIGS. 3-5.

Referring to FIG. 3, a protected light source 48 comprises a pair ofmulti-wavelength light sources (MWLSs) 50 optically connected torespective input ports a 2×2 3 dB optical coupler 52 having two inputports and two output ports. Each of the output ports 54 of the opticalcoupler 52 is connected to supply seed light to a respective set of oneor more WDM-PONs 2. A controller unit 55, which may include amicro-processor executing suitable software, controls operation of eachof the MWLSs 50.

FIGS. 4 a-c illustrate respective MWLSs 50 such as may be used in theprotected light source 48 of the present invention. In the embodiment ofFIG. 4 a. the MWLS 50 comprises a set of narrowband light sources 56,each of which is driven to generate a narrowband continuous wave (CW)light 58 having a center wavelength corresponding to that of apredetermined one of the wavelength channels of the WDM-PONs 2. Eachlight source 56 may be a conventional laser diode such as, for example,a distributed feed-back (DFB) laser, but this is not essential. Othernarrow band light sources, such as Fabry-Perrot lasers, DistributedFabry-Perrot Resonators, Passive Mode-Locked Bulk Lasers, ActiveMode-Locked Fiber Lasers and Dual-Beat Lasers, may be used. An opticalMUX 60 (such as, for example, a Thin-Film Filter—TFF) is used to combinethe CW light 58 from each light source 56 into a WDM seed light 62having a spectrum shown generally at 64. If desired, an opticalamplifier 66 (such as, for example, and Erbium Doped FibreAmplifier—EDFA) may be used to increase the optical power of the WDMseed light 62.

In the embodiment of FIG. 4 b, the MWLS 50 comprises a broadband lightsource 68 which is driven to generate a broadband continuous wave (CW)light 70 having a bandwidth corresponding with that of a predeterminedone of the channel bands of the WDM-PONS 2. For example, the bandwidthof the CW light 70 may correspond with either one of the C-band orL-Band channels of the WDM-PONs 2. If desired, the broadband lightsource 68 may be an Erbium Doped Fibre Amplifier (EDFA) generatingAmplified Spontaneous Emission (ASE), or a superluminescent lightemitting diode (LED) having a suitable spectral response, but this isnot essential. A comb filter 72 (such as, for example, a Thin-FilmFilter—TFF) is used to filter the CW light 70 to generate a WDM seedlight 74 having a spectrum shown generally at 76. As in the embodimentof FIG. 4 a, the WDM seed light 74 output from the comb filter 72comprises a plurality of narrowband CW lights, each of which has acenter wavelength corresponding to that of a predetermined one of thewavelength channels of the WDM-PONs 2. Here again, an optical amplifier66 (such as, for example, and Erbium Doped Fibre Amplifier—EDFA) may beused to increase the optical power of the WDM seed light 62, if desired.

FIG. 4 c illustrates a further alternative, in which the MWLS 50comprises a broadband light source 68 which is driven to generate abroadband continuous wave (CW) light 70 as described above. In thiscase, however, the broadband CW light 70, which has a spectrum showngenerally at 78, is output directly to the four-port 3 dB opticalcoupler 52. An optical amplifier 66 (such as, for example, and ErbiumDoped Fibre Amplifier—EDFA) may be used to increase the optical power ofthe broadband CW light 70, if desired. This embodiment relies on theMUXs 16 and 22 (FIGS. 1 and 2) of each WDM-PON 2 to provide the requiredfiltering such that a narrow band seed light is supplied to each of thetransceivers 6 and 28.

2×2 3 dB optical couplers are known in the art. In fact, mostcommercially available 3 dB optical power splitter/combiners areactually 2×2 devices, in which light injected into either one of the twoinput ports is evenly distributed between both of the two output ports.In the present invention, this fact is exploited to such that seed lightfrom an MWLS 50 connected to either input port will seed all of theWDM-PONs 2 connected downstream of the optical coupler 52. Thus, thecontroller 55 can designate one of the two MWLSs 50 as a “working” MWLS,and designate the other MWLS 50 as a “protection” MWLS. Recovery from afailure of the “working” MWLS can then be accomplished simply bydisabling the (failed) “working” MWLS while simultaneously enabling the“protection” MWLS. As may be appreciated, the controller unit 55 canreadily perform this operation at high speed. Because no opticalswitching is required to facilitate the protection switching function,optical losses associated with the presence of an optical switch areavoided. Similarly, because both output ports 54 of the four-port 3 dBoptical coupler 52 are coupled to a set of WDM-PONs 2, all of the seedlight output by any given MWLS 50 is used, so that the 3 dB opticalpenalty normally associated with the use of an optical coupler is alsoavoided.

In preferred embodiments, a respective protected light source 48 will beprovided to supply C-band and L-band seed lights to the WDM-PONs 2. Asmay be seen in FIG. 5, each of the WDM-PONs 2 can be topologicallysimilar to that illustrated in FIG. 1, except that the respectiveOptical Line Terminal (OLT) 4 of each WDM-PON 2 does not have its ownC-band and L-band seed light sources 34, 38 (see FIG. 1). Instead,C-band and L-band seed light for all of the WDM-PONs 2 is obtained fromrespective C-band and L-band protected light sources 48. Thisarrangement is particularly beneficial in a network node comprisingmultiple OLTs 4, each of which hosts a respective WDM-PON 2. Suchnetwork nodes typically are organized into one or more shelves, witheach shelf containing a plurality (e.g. 16 or more) OLTs. In thisapplication, a common protected light source 48 can be used to provideseed light to all of the OLTs 4 on a given shelf. In some embodiments, acommon protected light source 48 may be used to provide seed light toOLTs 4 on two or more shelves. In still further embodiments, a commonprotected light source 48 may be used to provide seed light to OLTs 4 indifferent central offices.

As may be appreciated, because the a common protected light source 48supplies seed light to multiple ONTs 4, the costs of the light source 48can also be shared across all of the involved WDM-PONs 2. Consequently,higher quality (and thus higher cost) components can be used in theprotected light source 48 than would be economical in an arrangement inwhich each OLT 4 has its own seed light sources 34, 38, as shown inFIGS. 1 and 2.

In the forgoing description, a 2×2 3 dB coupler 52 is used to distributeseed light from two MWLSs 50 to a plurality of WDM-PONs. Those ofordinary skill in the art will recognise that other passivecombiner/splitter devices may be used, without departing from thepresent invention. In fact, the coupler 52 can be provided as anypassive N×M coupler, in which light from any one of N input ports isevenly distributed to each of the M output ports. In the examplesdescribed above with reference to FIGS. 3-5, N=M=2. However, moregenerally, any coupler in which N≧2 and M≧2 can be used. It is notnecessary for N=M.

The embodiment(s) of the invention described above is(are) intended tobe exemplary only. The scope of the invention is therefore intended tobe limited solely by the scope of the appended claims.

1. A protected light source for an optical communication network,comprising: a passive optical coupler having at least two input portsand at least one output port, each output port being optically coupledto a respective branch of the optical network; at least two lightsources, each light source optically coupled to a respective input portof the optical coupler; and a controller connected to each light sourceand configured to cause at least one light source to supply light to thecoupler in a normal operation mode, and to cause at least one otherlight source to supply light to the coupler in a protection mode.
 2. Theprotected light source of claim 1, wherein each light source is amulti-wavelength light source.
 3. The protected light source of claim 2,wherein each light source is configured to emit light in a wavelengthband corresponding to a channel band of the optical communicationsystem.
 4. The protected light source of claim 2, wherein each lightsource is configured to emit light at a plurality of distinctwavelengths within a wavelength band corresponding to a channel band ofthe optical communication system, each distinct wavelength correspondingto a wavelength channel of the optical communication system.
 5. Theprotected light source of claim 4, wherein each light source comprises abroad band light source and a comb filter.
 6. The protected light sourceof claim 4, wherein each light source comprises a plurality of narrowband optical emitters, each optical emitter operable to emit at arespective discrete wavelength.
 7. The protected light source of claim1, wherein the controller is configured, in response to failure of theat least one light source supplying light to the coupler in normaloperation mode, to cause the at least one other light source to supplylight to the coupler in the protection mode.
 8. The protected lightsource of claim 7, wherein the controller is configured, in response tofailure of the at least one light source supplying light to the couplerin normal operation mode to disable operation of the at least one lightsource supplying light to the coupler in normal operation mode.
 9. Theprotected light source of claim 1, wherein the optical coupler hasmultiple output ports each for coupling to a respective branch of theoptical network.
 10. The protected light source of claim 1, wherein eachlight source is configured to supply seed light for a wavelengthdivision multiplexed passive optical network (WDM-PON).
 11. A method ofoperating a protected source of light for an optical communicationnetwork, the method comprising: optically coupling each of at least twolight sources to a respective input port of a passive optical coupler,the passive optical coupler having at least two input ports and at leastone output port, each output port being optically coupled to arespective branch of the optical network; controlling at least one lightsource to supply light to the coupler in a normal operation mode; andcontrolling at least one other light source to supply light to thecoupler in a protection mode.
 12. The method of claim 11, wherein eachlight source is a multi-wavelength light source.
 13. The method of claim12, wherein each light source is configured to emit light in awavelength band corresponding to a channel band of the opticalcommunication system.
 14. The method of claim 12, wherein each lightsource is configured to emit light at a plurality of distinctwavelengths within a wavelength band corresponding to a channel band ofthe optical communication system, each distinct wavelength correspondingto a wavelength channel of the optical communication system.
 15. Themethod of claim 14, wherein each light source comprises a broad bandlight source and a comb filter.
 16. The method of claim 14, wherein eachlight source comprises a plurality of narrow band optical emitters, eachoptical emitter configured to emit at a respective discrete wavelength.17. The method of claim 11, comprising controlling the at least oneother light source in response to failure of the at least one lightsource supplying light to the coupler in normal operation mode, to causethe at least one other light source to supply light to the coupler inthe protection mode.
 18. The method of claim 17, comprising controllingthe at least one light source in response to failure of the at least onelight source supplying light to the coupler in normal operation mode todisable operation of the at least one light source supplying light tothe coupler in normal operation mode.
 19. The method of claim 11,wherein the optical coupler has multiple output ports each for couplingto a respective branch of the optical network.
 20. The method of claim11, comprising operating the protected source of light to supply seedlight for a wavelength division multiplexed passive optical network(WDM-PON).